I've been wondering how hard it would be to do "poor-man's-fixed-point-gps" with two android devices. Put the devices on top of each other on the ground. This is your fixed point (wathever gps reading their each have, is "local zero").

Now pick one up. The one on the ground is your fixed point, the other unit is what you use for measuring. I wonder if doing it like that (possibly with two or three "anchors") -- would give increased precision? I haven't really looked in to what makes cheap gps "unreliable".

The units could communicate via bluetooth/mesh wifi for low latency, or over the internet (wifi/gprs/4g/dial-up...) -- if one just needed "point-to-point" measurements. Eg measuring a foundation or something related for construction.

I wonder if it would be "repeatable" (possibly only with the same units). And if one might get cm precision that way.

[ed: Not sure which part is "hard". Maybe (just for science experiment fun) one could drop a phone at one end of a 100m running track, an another at the other end - to get an exact 100m baseline (+/- 10cm?). I wonder if that might be useful in order to calibrate two or more units to work in pairs -- if there is some kind of predicable source of error in addition to just gps/signal noise etc].